1. Field of the Invention
The present invention relates to a measurement apparatus which measures a surface.
2. Description of the Related Art
The dimensions of the primary mirror of a telescope installed on the ground for astronomic observation are becoming larger to improve the performance of the telescope. For example, in the Subaru Telescope, a primary mirror formed from one mirror has a dimension of 8.2 m.
Recently, there has been proposed a telescope using, as a primary mirror, a composite mirror configured by connecting a plurality of hexagonal mirrors (segment mirrors). For example, the primary mirror of the TMT (Thirty Meter Telescope) tries to implement an effective aperture of 30 m by using a composite mirror formed from 492 segment mirrors (hexagonal mirrors each having a circumscribed circle diameter of 1.5 m and a diagonal line length of 1.44 m).
To manufacture a segment mirror forming such a composite mirror at high precision, the shape (surface shape) of a substrate for forming the reflecting surface (mirror surface) needs to be accurately measured. FIG. 8 is a schematic view showing the arrangement of part of a composite mirror. Referring to FIG. 8, six segment mirrors Ma, Mb, Mc, Md, Me, and Mf are arranged closely. The segment mirrors Ma to Mf are hexagonal mirrors each having a diagonal line length of 1.44 m. When forming a composite mirror by closely arranging a plurality of segment mirrors Ma to Mf, an unnecessary region of each segment mirror near the periphery (region where no surface shape can be measured) needs to be reduced to increase the effective aperture of the composite mirror. This requires a technique of measuring the surface shape of a segment mirror in a non-contact manner at pitches of several mm (2 to 3 mm) for a region of the segment mirror except for a peripheral region inward from the periphery by a width of about 1 mm.
As the technique of measuring the surface shape of such a large surface in a non-contact manner, Japanese Patent Laid-Open No. 2009-145095 discloses a three-dimensional shape measurement apparatus including a non-contact type probe using a double-pass interference method. In this measurement apparatus, as shown in FIG. 9, light from a light source LS travels toward a cube corner reflector CC, standard surface SS, test surface TS, and the like, and is detected by a detector DD, thereby measuring the shape of the test surface TS. At this time, the surface shape of the test surface TS can be measured in a non-contact manner by measuring a change of the optical path length of measurement light while driving the non-contact type probe within the X-Y plane.
As the technique of measuring the surface shape of a test surface in a non-contact manner at a high spatial frequency with a resolution of 1 mm or less, U.S. Pat. No. 4,353,650 discloses a three-dimensional shape measurement apparatus including a non-contact type probe using a heterodyne interference method. In this measurement apparatus, light of a frequency f1 and light of a frequency f2 emitted by a light source are separated by a Wollaston prism into light of the frequency f1 and light of the frequency f2, which are condensed by a condenser lens at different positions on a test surface. The measurement apparatus is configured to hold a test surface to be rotatable about the rotation axis, and to condense (that is, focus) light of the frequency f1 at a point on the rotation axis. By rotating the test surface about the rotation axis, of the entire test surface, a region on the circumference of a circle centered at the point where the light of the frequency f1 is condensed is irradiated with light of the frequency f2. The surface shape in the region can therefore be measured.
However, when the conventional measurement apparatus is applied to measurement of the surface shape of a segment mirror, the following problems occur. For example, in the measurement apparatus disclosed in Japanese Patent Laid-Open No. 2009-145095, as shown in FIG. 9, measurement light reflected by the test surface TS reciprocates twice between the standard surface SS and the test surface TS. If the diameter of measurement light is decreased, an overlap with reference light necessary to obtain a heterodyne signal decreases, failing in obtaining a heterodyne signal of a satisfactory strength. To prevent this, measurement light has a diameter of at least about 3 mm. While measurement light reciprocates twice between the standard surface SS and the test surface TS, the first light and the second light reciprocate at a distance of about 6 mm. With such measurement light formed from two beams which are spaced apart from each other by 6 mm and have a diameter of 3 mm, it is difficult to measure the surface shape of a segment mirror at pitches of several mm. Further, with such measurement light, a surface shape in a region of the segment mirror near the periphery cannot be measured.
In the measurement apparatus disclosed in U.S. Pat. No. 4,353,650, light of the frequency f2 is condensed on a test surface, so the surface shape can be measured at a resolution of 1 mm or less. However, the measurement apparatus can measure only a surface shape in a region on the circumference of a circle centered at a point where light of the frequency f1 is condensed. For this reason, the measurement apparatus disclosed in U.S. Pat. No. 4,353,650 cannot measure a surface shape at pitches of several mm in an arbitrary region of a segment mirror except for the peripheral region. The test surface needs to fall within the range of the depth of focus of the condenser lens for condensing light of the frequency f2. It is therefore very difficult to measure the surface shape of a test surface having a curvature, such as a segment mirror.